PROJECT SUMMARY A conventional prosthetic ankle consists of a carbon fiber ankle-foot complex that is nominally configured to a neutral position. These prostheses generally work well for level walking and level-ground standing, but lack the adaptability to explicitly accommodate other terrain or locomotion activities, such as slope walking, slope standing, and stair ambulation. As such, common activities with a conventional prosthesis can be some of the most daunting and dangerous for individuals with lower limb amputation. The investigators have developed a novel microprocessor-controlled multifunction ankle prosthesis that is able to adapt its behavior to accommodate slope walking, slope standing, and stair ambulation in a compact, lightweight, quiet, cost-competitive device that requires little electrical power. The prosthesis provides three microprocessor-controlled behaviors ? a selectable stiffness equilibrium angle, lockable conformal damping, and swing-phase repositioning. No other prosthesis combines this set of features. The investigators hypothesize that the proposed prosthesis will provide a number of mobility, stability, and health benefits to individuals with transtibial amputation, relative to a conventional prosthesis. The primary goal of this Phase I proposal is to assess some of the prospective biomechanical benefits of the proposed prosthesis relative to a conventional prosthesis in order to evaluate the value proposition offered by it. In order to do so, the investigators propose herein the following three aims: 1) develop and refine a level ground walking controller for the novel prosthesis; 2) conduct biomechanical studies on three subjects with transtibial amputation that characterize the efficacy of the novel prosthesis during level-ground walking, relative to walking with a conventional prosthesis; and 3) develop a stair descent controller, and conduct biomechanical experiments that characterize on at least one subject with transtibial amputation the efficacy of the novel prosthesis during stair descent, relative to stair descent with a conventional prosthesis. If the prosthesis is shown to provide substantive value to the user relative to conventional prostheses and other devices currently available, the investigators will proceed to a Phase II proposal in support of making the prosthesis commercially available to individuals who may benefit from it.